Yellowstone

The world's first national park -- and perhaps the most famous in all of America -- Yellowstone covers an astonishing 3,400 square miles and encompasses a dizzying array of animal and bird species, including such large, free-roaming mammals as bison, elk, grizzly bears and wolves. Beyond the usual offerings of camping, hiking and sightseeing, visitors can opt to take bus tours in the summer and snowmobile tours in the winter. Those looking to do some mountain climbing should look elsewhere, as the region's volcanic rock is notoriously unstable.